The phenomenon that sound spreads in the open air where reflective objects do not exist is called a free sound field. The free sound field is not generated indoors due to the reflection of sound on the wall. An anechoic chamber is a room designed to have conditions similar to the free sound field by installing an absorber or an absorbing block having a high sound absorbing ratio on walls, ceilings and/or floors of the room so as to prevent the sound generated in the room from being reflected through the walls, the ceilings and/or the floors.
The anechoic chamber can be used in a wide range of industrial fields such as aircraft parts, automobile parts, electric appliances, communication devices and medical devices. It can be also used as a space for precise measurement of sound associated with a speaker, a microphone or the like in acoustic makers or acoustical research institutes.
The sound absorber installed in the anechoic chamber absorbs noises generated by the operation of the product in the anechoic chamber, thereby preventing noises from being reflected. For preventing the reflection of noises, the sound absorber used in the anechoic chamber should have a very high sound absorbing ratio, usually the normal incidence sound absorbing ratio of 0.99 or more, and conservatively the normal incidence sound absorbing ratio of 0.9 or more.
The common absorbers used in the anechoic chamber are wedges having a triangular shape protruding toward the inside of the anechoic chamber.
FIG. 1 is a schematic view of an example of wedges, and FIG. 2 is a schematic view of an example of an anechoic chamber where the wedges are installed.
Referring to FIG. 1, a wedge 1 used in the common anechoic chamber may be formed of an absorbing material. Examples of the absorbing material may include a porous sound absorber such as glass wool, foamed urethane and the like. The wedge 1 may be formed on walls, a ceiling and/or a floor of an anechoic chamber 10 and exhibit the sound absorbing effect. Referring to FIG. 2, the anechoic chamber 10 may include a square pipe 3 formed on the at least one wall, the ceiling and the floor in a grid shape, a sound insulating panel 5 formed in a space between the square pipes 3 and a wedge 1 formed on the sound insulating panel 5.
A length L of the wedge 1 may be determined according to the cut off frequency to create a free sound field and be usually ¼ wavelength. The length of the wedge 1 required according to the cut off frequency is calculated as follows:
TABLE 1Cut off frequency (Hz)40506380100125163200250315¼ wavelength (m)2.11.71.351.060.850.680.520.430.340.27(length of the wedge)
However, since the maximum length of the wedge 1 is about 1.5 m, it is impossible to fabricate the absorber at the length longer than about 1.5 m used as the wedge 1. Further, the wedge 1 having the length longer than about 1.5 m cannot be practically used because it is bent due to its own weight after installation on the wall. In case of the wedge 1 having the maximum length of about 1.5 m, the cut off frequency is limited to about 63 Hz. Therefore, it is impossible to implement the sound absorbing effect at the frequency lower than about 63 Hz by using the wedge 1. As a result, it is difficult to form the anechoic chamber having the cut off frequency of 50 Hz or less which is required for an automobile field or an engine field by using the common wedge 1. Moreover, if the length of the wedge 1 becomes longer, there may be a problem that the space occupied by the wedge is increased and the actual available space is reduced.
Meanwhile, the sound absorber may be classified into three types, that is, a porous sound absorber, a resonance sound absorber and a plate-type sound absorber according to the method and the technical feature of sound absorption.
The porous sound absorber has a plurality of small bubble-like pores or a fibrous structure on the surface of the absorber and inside the absorber, so that it is vibrated by the sound waves of air. As a result of such vibration, friction between materials may occur and sound energy may be converted into heat energy and absorbed. The main sound absorption range of the porous sound absorber may be the high-frequency range of 250 Hz or more. The porous sound absorber may be formed of glass wool, rock wool, foamed resin materials, and fabrics, and so on.
The resonance sound absorber uses a principal of Helmholtz Resonators and is a container of gas (usually air) with an open hole (or neck or port). A volume of air in and near the open hole vibrates because of the ‘springiness’ of the air inside. When the sound wave with a resonance frequency arrives, it is possible to absorb acoustic energy due to viscous resistance of the open hole. The main sound absorption range of the resonance sound absorber may be the middle frequency range of 125 Hz to 250 Hz. The resonance sound absorber may include a perforated plate having an air layer formed on the rear side of the plate.
The plate-type sound absorber uses resonance due to vibration of the plate. The plate-type sound absorber may exhibit the sound absorbing effect by converting acoustic energy into vibrational energy when a sound wave vibrates the plate. The main sound absorption range of the sound absorber using resonance due to vibration of the plate may be the low frequency range of 125 Hz or less. The plate-type sound absorber may be formed of a metal plate, a vinyl film, and a gypsum wallboard, and so on.
The wedge used in the anechoic chamber may be formed of the porous sound absorber. In this case, in order to increase the sound absorbing ratio at the cut off frequency of 100 Hz or less, the length of the wedge should become longer. However, considering the limited size of the anechoic chamber, increasing the length of the wedge is limited so that it is practically impossible to achieve the sound absorbing effect in the low frequency region with the porous sound absorber.
In case of the resonance sound absorber, theoretically, in order to obtain the sound absorbing effect in the low frequency region, the lower the frequency, the thicker the air layer behind the perforated plate. However, like the porous sound absorber, considering the limited size of the anechoic chamber, increasing the thickness of the air layer is limited so that it is practically impossible to achieve the sound absorbing effect in the low frequency region with the resonance sound absorber.
Meanwhile, the plate-type absorber may be used for sound absorption in the low frequency region and the size, thickness, material and the like are the main factors that determine the sound absorbing effect. A resonance frequency f0 can be calculated as follow:
      f    0    =            c              2        ⁢        π              ⁢                  ρ        ML            
wherein:
M: Mass per unit area of plate (kg/m2),
L: Thickness of rear air layer (m),
ρ: Density of air (kg/m3),
c: Sound velocity of air (m/s).
According to the above formula, the resonance frequency f0 of the plate-type absorber may vary depending on the mass per unit area of plate M, and the mass per unit area of plate M may vary depending on the thickness of the plate. Accordingly, in order to adjust the resonance frequency of the plate-type absorber, it is required to change the thickness and size of the plate-type absorber. However, since the plate is usually formed of metals, increasing the weight and size is limited so that controlling the resonance frequency is limited.
Patent Literature 1 discloses a plate-type absorber that uses resonance due to vibration of the plate, and Patent Literature 2 discloses an anechoic chamber including such a plate-type absorber.
However, since the plate-type absorber disclosed in Patent Literatures 1 and 2 uses resonance due to vibration of a metal plate, the weight or size of the metal plate is limited so that controlling the resonance frequency is limited.
Moreover, the plate-type absorber disclosed in Patent Literatures 1 and 2 includes the metal plate and a rear polymer board. The metal plate should be bonded to the rear polymer board via adhesion and the like. However, since most porous sound absorbers used as the rear polymer board tend to fall off easily due to the weight of the rear polymer board itself when attached to the metal plate, the sound absorber used as the rear polymer board that can be bonded to the metal plate is very limited. For this reason, in Patent Literatures 1 and 2, the rear polymer board that can bonded to the metal plate is limited to melamine resin foam that can prevent falling due to the weight.
Further, in the plate-type sound absorber, the edges of the metal plate should be fixed consistently. If the porous sound absorbers located on or below the metal plate press the metal plate, the sound absorbing ratio may change. As a result, when the plate-type absorbers are installed on the walls and on the ceilings or floors, respectively, they may exhibit different sound absorbing ratios.
Therefore, it is still required to develop techniques for a novel anechoic chamber that can overcome drawbacks of the common wedges or plate-type absorbers using the metal plate.